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Tech Article

Audi C5 A6 Quattro 4.2 V8 Fuel Rail Pressure

Last Updated: 09/24/2017
Parts Group: Fuel rail
This article discusses the fuel pressure regulator for the Audi C5 A6 with the 4.2 V8 as made from 1999 to 2004, with the main focus being on how we tested for fuel pressure, and how we decided to replace the fuel pressure regulator.

On our project car, a silver 2000 Audi A6 Quattro 4.2 V8, the engine needs to be cranked for perhaps ten seconds before it starts. My BMW 528e has a fuel pump without a check valve, and it shows the same symptom. Essentially, the fuel rail is pressurized correctly while the engine and fuel pump are running, but once the engine is turned off, and the pump loses power, the fuel slowly drains back into the fuel tank, and then there's no pressure in the fuel rail. If I restart the engine while there is still some fuel in the fuel line, the car starts more rapidly than if all the fuel has drained away.

My tech and I debated whether the Audi has the same cause (lack of fuel pressure) so I decided to test. I removed the flexible black rubber ducting that guides the air flow downstream of the air filter box. This required a large flat-headed screwdriver to loosen the two hose clamps (one at each end) and some gentle finagling to get the ducting wiggled out of position. This provided access to the metal fuel line as it connects to the fuel rail. The connection has two nuts; one is a 14mm nut that I held with a wrench. This 14mm nut is immovably integrated with the fuel rail and is NOT intended to be turned. If you turn it, you can damage the fuel rail. If you do, no worries, we sell used units: The 14 mm wrench basically holds the nut in position to prevent strain on the fuel rail when I loosen the other nut, for which I used a 17mm wrench. On the silver car, loosening the 17mm nit had no effect, and so we retightened that nut. Then we did the same thing on a black 2000 Audi A6 Quattro 4.2 V8 car that starts near-instantly when the key is turned. On that car, as soon as I loosened the 17mm nut, I heard the sound of fuel escaping at high pressure, and I smelled fuel. Point proved. And yes, all the common-sense safety precautions applied: safety glasses, special gloves, cold engine, fire extinguisher in the hands of a capable person standing a few feet away, and working outside away from spark or flame.

I suppose that a fuel pressure regulator could hold fuel pressure at 4.0 bar (its official rating) yet also have a slow leak too, that causes the fuel to drain back into the tank. So, replacing that seemed logical too. We did so, using a new Bosch unit, but the problem is still persisting. Next suspect: the fuel pump, which is supposed to have a check-valve to prevent fuel from exiting the fuel line.
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